The present invention relates to the lighting arts. It especially relates to resonant cavity light emitting devices such as resonant cavity light emitting diodes (RCLED's) and vertical cavity surface emitting lasers (VCSEL's), and will be described with particular reference thereto. However, the invention will also find application in conjunction with other group III-nitride based light sources.
One of the major challenges with solid state lighting at present is efficient extraction of light from the light emitting diode. Another challenge is that light emitting diodes typically emit only one wavelength, whereas for many applications at least one additional wavelength or white light would be desirable. The use of phosphors, typically in a polymer matrix, allows generation of additional wavelengths and/or white light, but often entails loss of efficiency by Stokes shifts, reflective or light-scattering losses, and device failures associated with the phosphor packaging. Resonant cavity devices offer a way to provide more efficient light extraction, but it has so far been difficult to fabricate reliable resonant cavity devices in the group III-nitride material system.
Those skilled in the art recognize substantial advantages of growing group III-nitride light emitting devices on a large-area gallium nitride wafer. Advantages of the gallium nitride substrate material typically include: (i) a close lattice match which, neglecting dopant effects, is essentially perfect for gallium nitride device layers; (ii) reduced strain and dislocation formation in the epitaxial group III-nitride layers as a consequence of the close lattice match; (iii) chemically abrupt interfaces without problematic interdiffusion; (iv) substantial elimination of anti-phase boundaries; and (v) thermal matching that promotes thermal stability during cooldown from high epitaxial growth temperatures and during high temperature device processing. These advantages are particularly significant for large area devices and large-area epitaxial wafers, where lattice mismatch strain or thermal stresses can lead to wafer bowing or breakage.
Advantages of growing on a large area gallium nitride wafer, as opposed to a small area gallium nitride substrate, typically include: (i) economy of scale (more devices per wafer); (ii) easier handling; (iii) easier automated machine manipulation; and (iv) the ability to fabricate large-area devices. The latter advantage is particularly useful for light emitting devices. In many illumination applications, for example, light emitting devices are preferably large compared with typical microelectronic devices. Resonant cavity light emitting diodes for illumination applications, for example, preferably are of order hundreds of microns on a side or larger, corresponding to device areas in the tens of thousands of square microns or higher. These device areas place a lower limit on the feasible size of the substrate.
In spite of these well-known advantages, commercial group III-nitride light emitting devices generally continue to be grown heteroepitaxially on sapphire or silicon carbide substrates due to a lack of high quality large-area gallium nitride substrates. The chemical passivity of nitrogen, a high melting temperature of gallium nitride, and other factors have heretofore made growth of large volume and high quality gallium nitride boules problematic.
U.S. Pat. Nos. 5,637,531 and 6,273,948 disclose methods for growing gallium nitride crystals at high pressure and high temperature, using liquid gallium and gallium-based alloys as a solvent and a high pressure of nitrogen above the melt to maintain GaN as a thermodynamically-stable phase. The process is capable of growing electrically-conductive GaN crystals with a dislocation density of about 103–105 cm−2 or, alternatively, semi-insulating GaN crystals with a dislocation density of about 10–104 cm−2, as described by Porowski, “Near defect-free GaN substrates” [MRS Internet J. Nitride Semicond. Research 4S1, G1.3 (1999)]. However, the conductive crystals have a high n-type background doping on the order of 5×1019 cm−3, believed to be due to oxygen impurities and nitrogen vacancies. The high n-type background causes substantial crystal opacity, for example optical absorption coefficients of around 200 cm−1 in the visible range, which is problematic for flip-chip light emitters, and causes the lattice constant to increase by about 0.01–0.02%, generating strain in epitaxial GaN layers deposited thereupon. The undoped GaN substrates formed by this method have a rather limited carrier mobility, about 30–90 cm2/V-s, which may be problematic in high-power devices.
Another technology for growth of pseudo-bulk or bulk GaN is hydride/halide vapor phase epitaxy, also known as HVPE. In one approach, HCl reacts with liquid Ga to form vapor-phase GaCl, which is transported to a substrate where it reacts with injected NH3 to form GaN. Typically the deposition is performed on a non-GaN substrate such as sapphire, silicon, gallium arsenide, or LiGaO2. The dislocation density in HVPE-grown films is initially quite high, on the order of 1010 cm−2 as is typical for heteroepitaxy of GaN, but drops to a value of about 107 cm−2 after a thickness of 100–300 μm of GaN has been grown. Heteroepitaxial growth of thick HVPE GaN results in strain-induced bowing during cooldown after growth, which remains even after removal of the original substrate.
In view of the substantial difficulty in producing large gallium nitride boules, some efforts have been directed toward developing complex techniques such as epitaxial lateral overgrowth (ELO) for producing individual gallium nitride substrates. In ELO, an epitaxy-inhibiting mask is deposited over a nucleation substrate such as a sapphire wafer. The mask is lithographically processed to define openings. Gallium nitride growth nucleates in and fills the openings, and then grows laterally over the masked areas in a lateral overgrowth mode. ELO material has been shown to have suppressed dislocation densities. Optionally, the nucleation substrate is removed and the ELO growth process is repeated on the free-standing gallium nitride wafer. Some reports claim dislocation densities as low as 104 cm−2 obtained by ELO.
However, much higher dislocation densities remain above the openings where ELO growth initiates. Moreover, coalescence of lateral overgrowth from adjacent openings produce tilt boundaries that typically manifest in thick layers as arrays of edge dislocations. Repeated application of epitaxial lateral overgrowth is not expected to substantially suppress the tilt boundaries. Thus, epitaxial lateral overgrowth is not upwardly scalable in the lateral wafer dimension, and usable growth dimensions are limited to about the order of the spacings of the nucleation openings. Furthermore, ELO does not produce a three-dimensional single-crystal boule, and the processing involved in producing each ELO gallium nitride wafer is labor-intensive, making automation of the ELO wafer formation process difficult.
Doping of GaN by rare earth metals is known to produce luminescence. For example, Lozykowski et al. (U.S. Pat. No. 6,140,669) disclose incorporating rare earth ions into GaN layers by ion implantation, MOCVD, or MBE, and annealing at 1000° C. or greater. Birkhahn et al. (U.S. Pat. No. 6,255,669) disclose fabrication of light-emitting diodes using GaN layers doped with a rare earth ion or with chromium. However, these references focus on thin GaN epitaxial layers rather than bulk crystals and do not relate to resonant cavity devices.
Mueller-Mach et al. (WO 01/24285 A1) disclose the fabrication of GaN-based light-emitting diodes on a single crystal phosphor substrate, preferably, rare-earth-doped yttrium aluminum garnet. DenBaars et al. (WO 01/37351 A1) disclose the fabrication of GaN-based light-emitting diode structures, including a vertical laser structure, on a substrate doped with chromium or other transition or rare earth ions. However, the disclosed laser structure employs only a single cavity and has no capability for directional emission of two or more visible wavelengths of light or of white light.
The present invention contemplates an improved apparatus and method that overcomes the above-mentioned limitations and others.